1. Field of the Invention
The present invention relates to an optical transmission apparatus which transmits data by means of light for the purposes of, for example, increase of the data rate between boards, chips, or the like, and reduction of electromagnetic noises.
2. Description of the Related Art
Recently, attention is given to an intrasystem optical connection technique which is called short-distance optical interconnection. For example, intraboard optical interconnection which connects semiconductor elements with each other via optical wiring receives attention. An electro-optic circuit board in which a transparent medium having a function of optical wiring is disposed on a printed circuit board for electric wiring has been proposed.
JP-A-2002-62457 is a prior technical literature related to the invention.
FIG. 8 shows an optical signal transmission device (hereinafter, refereed to as “optical transmission path”) 3 which is disclosed in JP-A-2002-62457. Stair-like steps are formed on a transparent medium 30 which is made of inorganic glass or a plastic material such as polymethyl methacrylate, polycarbonate, or amorphous polyolefin. A reflective light diffusing section 31 is formed on one end face of the transparent medium 30. On the ends of the steps, 45-degree faces 32A, 33A, 34A, and 35A are respectively disposed. Their upper faces are optically coupled with a light emitting element and a light receiving element to function as entering and emitting faces for a light signal.
FIG. 8A shows optical transmission in the transparent medium 30 in the case where a light signal L enters from the upper face of the 45-degree face 35A. The light signal L propagates through the transparent medium 30, and reflected and diffused by the reflective light diffusing section 31 as shown in the figure, and then reflected by the 45-degree faces 32A, 33A, 34A, and 35A toward their upper faces to be emitted therefrom.
FIG. 8B shows optical transmission in the transparent medium 30 in the case where a light signal L enters from the upper face of the 45-degree face 32A. The light signal L propagates through the transparent medium 30, and reflected and diffused by the reflective light diffusing section 31 as shown in the figure, and then reflected by the 45-degree faces 32A, 33A, 34A, and 35A toward their upper faces to be emitted therefrom.
When the optical transmission path 3 having the transparent medium 30 is used as an optical data bus, a light signal can be rapidly transmitted without producing a signal delay caused by the capacitances between electrical connection wirings and the resistances of the wirings.
In the case where optical transmission is to be performed at a high speed, a high-pass filter is usually used in a reception section in order to enhance the noise resistance and extract modulated components of the light intensity. In such a configuration, when signals of the same code are continuously input into the reception section for a given time period or longer, the quantization level becomes unstable and the signal recognizability is lowered. Moreover, there arises a disadvantage that a long time period must elapse before the signal recognizability is returned to the normal one. Therefore, a signal must be adequately coded so that the same code is not continuous.
As an example of such a coding method, known is the 8B10B coding method which is employed in standards such as Fiber Channel Standard. In the 8B10B coding method, a set of 8-bit signals is converted into a 10-bit signal in accordance with a given rule, and the signal is serialized in 10 to 1, whereby the mixture ratio of 1 and 0 is made close to 50%.
According to the 8B10B coding process, in a many-to-many communication in which a plurality of circuit boards (hereinafter, referred to as “nodes”) each having an optical communication section consisting of a light emitting element and a light receiving element are optically connected to one another via the optical transmission path 3 shown in FIG. 8, a light signal which is coded in a transmission section is emitted to be transmitted, and the light signal received by a receiving section is decoded, whereby high-speed data transmission is enabled. In this configuration, an AC-like light signal always flows through the optical transmission path 3, and hence the receiving section can be always set to an active state.
In such a conventional optical transmission apparatus, there is the possibility that, in a many-to-many communication, light signals are emitted from plural nodes at the same time. Consequently, there arises the problem in that, when the timings of the emissions coincide with one another, the light signals the amount of which exceeds an allowable light amount of the receiving section enter the receiving section, thereby damaging the receiving section or lowering the signal recognizability.